1. Field of the Invention
The present invention relates to an arrangement for discriminating whether or not a semiconductor laser is functional and, more particularly, to a technique of determining whether or not a semiconductor laser employed in a transmitting equipment of a light transmission system is functional under a condition that the laser is kept incorporated into the system.
2. Description of the Related Art
In general, a semiconductor laser employed in a transmitting equipment of the light transmission system is modulated by current pulses of high speed, such as 2 Gb/s, and, usually, oscillated on a single wavelength. In such case, it is known that the wavelength of emission spectrum is shifted from the dominant mode to the next mode due, for example, to a change in temperature, a change in level caused by a rise of the current pulse, or the like. This phenomenon is known as a "mode hopping" or "mode jump".
When mode hopping occurs, an error arises in that, in the position at which a correct signal, e.g., data of "0", should appear, an incorrect signal, i.e., a pseudo-pulse of "1", appears due to a chromatic dispersion of an optical fiber. Although the error caused by the mode hopping indeterminately occurs, a frequency of occurrence of the error, hereinafter referred to as an error rate, exceeding a certain demanded error rate is not preferable from the viewpoint of the realization of a satisfactory transmission characteristic in the light transmission system. That is, the lower the error rate, the better the light transmission system.
In practical use, however, errors due to the mode hopping rarely occur, and it is difficult to clarify the cause thereof when they do occur. When mode hopping occurs in the semiconductor laser at a rate above the demanded error rate, the measurement of the rate of the mode hopping has heretofore been difficult. Furthermore, it has been difficult to properly select a semiconductor laser by measures taken in advance. Accordingly, the measurement of the error rate has inevitably been carried out under the condition that the laser is incorporated into the light transmission system.
In this type of measurement, however, even if an error is detected in the receiving system, it is impossible to exactly discriminate whether the error is caused by the semiconductor laser or by the transmission line such as an optical fiber line. Namely, as stated above, it has been hard to make clear what caused the occurrence of the error. Therefore, when an error occurs with an error rate above the demanded error rate in a conventional light transmission system, in order to find out whether the cause of the error is due to the mode hopping of the laser, it becomes necessary to make an adjustment for all of the main elements constituting the whole system including the transmitting and receiving system for maintenance of the quality of the transmission characteristic to a constant level. This makes works or operations extremely troublesome or complicated and thus is not preferable.